A 20-year-old YouTube prankster has admitted to fatally shooting her boyfriend in a videotaped stunt they hoped would go viral.

Monalisa Perez told investigators she fired a handgun from about a foot away at a book held by Pedro Ruiz III in front of his chest on June 26, BBC reports.

She said the couple, who have a three-year-old daughter together and another on the way, thought the bullet would be caught by the book's volume.

However, Ruiz, 22, was killed when the gunshot went into his chest.

Monalisa Perez. Image: Police Handout.

Perez pleaded guilty to second-degree manslaughter on Friday, after having already served six months in prison.

According to court records, the couple were seeking internet fame from the stunt which went so wrong.

Their previous stunts included Ruiz falling out of a tree or jumping into a swimming pool from the top of a house. Just hours before the fatal stunt, Perez had tweeted that she and Ruiz were going to "shoot one of the most dangerous videos ever".

Ruiz would often play "pranks" on her boyfriend in videos

Perez told investigators Ruiz had convinced her to do the stunt by showing her other books he had shot where the bullet did not go through the other side.

They had then used a GoPro camera to film as Ruiz clutched the 4cm thick hardcover encyclopedia to his chest. The couple's daughter and nearly 30 onlookers had been watching when Perez fired the .50 calibre handgun at Ruiz's chest.

Perez, who was heavily pregnant with the couple's second child, called 911 to report she had accidentally shot her boyfriend. Ruiz had a single gunshot wound in his chest and was pronounced dead at the scene.

Perez will be sentenced in February.

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Ruiz's aunt, Claudia Ruiz, previously told WDAY-TV Ruiz wanted to do the stunt "because we want more viewers, we want to get famous."
"He had told me about that idea and I said, 'Don't do it. Don't do it. Why are you going to use a gun? Why?'" Claudia Ruiz told the TV station.
"They were in love, they loved each other," she said. "It was just a prank gone wrong. It shouldn’t have happened like this. It shouldn’t have happened at all."
Perez had started the vlog earlier this year, and had uploaded many videos including pranks with herself and Ruiz. Earlier this week, she tweeted that they were planning to shoot a “dangerous” video.

The couple’s child is expected in September; a baby boy they decided to name Pedro, after his father.